It has heretofore been proposed that ultrasonic measuring techniques be employed for monitoring or measuring the level of material in a storage vessel. As shown in U.S. Pat. No. 4,000,650, for example, an ultrasonic transducer may be mounted at the top of a storage vessel and directed to transmit pulsed energy downwardly toward, and receive echo signals reflected upwardly from, the upper surface of the stored material. The transmitted and echo signals travel through the headspace or air between the transducer at the top of the container and the material surface. Digital electronics are employed to sample the echo signals returned to the transducer, and to distinguish a true echo signal from the surface of the material from false echo signals due to noise, etc. by integrating the echo signals associated with eight successive transmitted pulses so that the sum of the true echo signals returned from the material surface is substantially greater than the sum of random noise signals. Material level is indicated by determining the round-trip transit time of the transmit/echo sequence associated with the true echo signal.
Although the system disclosed in the noted patent addressed a number of problems theretofore extant in the art, further improvements remain desirable. For example, typical prior-art ultrasonic material level measurement systems employ a fixed frequency oscillator that is switched on and off by control circuity for driving the transducer. Oscillator frequency and duty cycle, and usually total burst duration time, are set at the factory and assumed to remain constant. Transducer resonant frequency, for example, may shift with changes in temperature; however, no accommodation is made for changing the frequency of the drive signal. Another problem in typical prior-art devices is associated with gain or amplification applied to the echo signals. Although signal strength is attenuated as a function of the square of the distance traveled through the container headspace, amplification of echo signals is typically held constant. Thus, amplitude of a true echo signal at the processing circuitry from a distant material surface may be substantially less than amplitude of noise signals received earlier in time.
A general object of the present invention is to provide a system and method for ultrasonic measurement of material level that employ state-of-the-art microprocessor-based electronics for enhanced economy of manufacture, as well as improved reliability and versatility in the field. Another object of the present invention is to provide a system and method of the described character in which the level measurement is automatically compensated for variations in temperature in the air through which the signals travel, which affect velocity of the ultrasonic signals through the air, and thereby provide improved resolution and accuracy in the measurement results. Yet another object of the present invention is to provide a system and method of the described character that embody an improved technique for distinguishing true echo signals returned from the material surface from false signals due to dust, noise or other transient occurrences.